When creating cyclic patterns in masks or reticles for the production of for instance displays, such as TFT-LCD's or plasma-displays, a key quality requirement is the absence of defects, such as shade differences, light and dark fields, stripes or lines in said pattern.
Deviations, e.g. CD (critical dimension) or positioning errors, causing said defects are usually very small, from a few hundred nanometers down to a single nanometer. Deviations of that size spread over a relatively large area on a display photo mask, which may be 1500×1200 mm, may be very difficult, for not saying impossible, to detect by measuring. Unfortunately a human eye is very sensitive to systematic changes and therefore can detect such small deviations as stripes in the image. The human eye is extremely sensitive to periodical intensity variations in the image. The viewing distance will influence the appearance of said periodical intensity variations. Generally speaking, periodical intensity variations will be detectable by the human eye if the difference in contrast is around and above 0,5% in a spatial frequency range of 1-20 mm. For normal viewing distances periodical intensity variations below about 1 mm will not show up.
There are some methods known in the art to reduce or eliminate the appearance of defects in a photo mask. The methods known in the art are especially suitable to reduce or eliminate the appearance of periodical defects, which is the most common defect.
Periodical defects are caused by the beating frequencies between a pattern pitch and a system pitch in a certain direction. The pattern pitch is defined as the distance between equal features in the pattern. The pattern pitch may be different in an X and a Y direction of said pattern. One system pitch in a raster scanning system is a Y-pitch, defining a distance between two adjacent center of gravity of exposure spot sizes along a sweep direction of said exposure beam. A numerous exposure spots, in the range of several hundreds, along the sweep direction are forming a scan line. The exposure spot is preferably continuously on within a scan line, but may be switched off at any given time determined by a frequency of a clock generator connectable to a modulator which is modulating said exposure beam according to desired pattern data. Another system pitch is an X-pitch, defining a distance between two adjacent parallel scan lines of said exposure beam. A numerous scan lines in X-direction are forming a strip. Strips stitched together will form a desired pattern on the workpiece.
One compensation method for eliminating or reducing periodical defects in the pattern on the workpiece is called scaling, which means that the distance from one feature to another feature in the pattern, i.e., the pattern pitch in X and Y direction, should be an integer multiple of the system pitches. If there is a mismatch in one or two directions, the pattern is rescaled in one or two directions to match said system pitches. Pattern size is maintained by adjusting a scale reference in the machine, for instance by changing the wavelength of an interferometer or resealing an initial interferometer scale by a desired factor, which interferometer functions as a position measuring device in said direction, see U.S. Pat. No. 5,635,976.
Unfortunately, when correcting the pattern as described herein above, masks or reticles patterned by using multiple exposure beams for increasing the writing speed may present certain side effects such as increased CD (Critical Dimension)-error. i.e., lines or features printed on a workpiece have less uniform line widths.
What is needed is a method and apparatus, which is capable to use the above mentioned compensation methods for periodical defects without increasing CD-error when creating a pattern on the workpiece by using multiple exposure beams.